Downsizing electronic devices, and making them lightweight have been progressing, so secondary batteries whose energy densities are higher have been desired as their electric power source. A secondary battery is one which takes out chemical energy, which a positive-electrode active material and negative-electrode active material possess by means of chemical reactions via an electrolyte, to the outside as electric energy. In such secondary batteries, lithium-ion secondary batteries are secondary batteries that possess the highest energy density among those which have been put into practical applications.
In lithium-ion secondary battery, lithium-containing metallic composite oxides, such as lithium-cobalt composite oxides, have been used mainly as an active material for the positive electrode; whereas carbon materials having multi-layered structures, which are capable of inserting lithium ions between the layers (i.e., the formation of lithium intercalation compound) as well as releasing lithium ions from between the layers, have been used mainly as for the negative electrode.
A polar plate for the positive electrode and negative electrode is made as follows. Each of the active materials, and a binder resin are dispersed in a solvent to make a slurry. This slurry is coated onto each of the metallic foils, namely, the current collectors. Subsequently, the solvent being included in the slurry is dried to remove in order to form each of the active-material layers on the current collectors, respectively. Then, the active-material layer and current collector are compression-molded by a roller-pressing machine, thereby completing a positive-electrode polar plate or negative-electrode polar plate.
Recently, as a negative-electrode active material for lithium-ion secondary battery, the development of next-generation negative-electrode active materials possessing charging/discharging capacities that greatly exceed the theoretical capacity of carbon material has been progressing. For example, materials including metals, such as Si and Sn that are capable of alloying with lithium, have been expected therefor.
In a case where Si or Sn, and the like, is used for an active material, these materials exhibit large volumetric changes being accompanied by the absorption/release of lithium at the time of charging/discharging. Consequently, it is difficult to satisfactorily maintain the bonding state between these materials and current collectors. These materials expand and contract repeatedly during charging/discharging cycles, as being accompanied by the absorption and release of lithium. Because of such expansions and contractions of active material, the active-material particles have been pulverized, or the active-material particles have fallen down from current collector. Because of the pulverization of active material, or because of the falling-down from current collector, the resulting cyclability of lithium-ion secondary battery becomes worse considerably.
Regarding those in which a carbon material is used as the active material, methods for suppressing the active material from falling down from current collectors have been investigated. Moreover, in active materials for positive electrode as well, methods for suppressing the active materials from falling down from current collector have been investigated.
For example, Patent Literature No. 1 sets forth an electrode for non-aqueous-electrolyte secondary battery being characterized in that, in an electrode being completed by laminating an electrode mixture-agent paste, which is made by kneading an electrode raw-material powder together with a binder as well as a solvent, onto a current collector in order to form an electrode mixture-material layer thereon, a binder concentration within the electrode mixture-agent layer is made so as to be more concentrated near the current collector. It discloses electrodes, which used polyvinylidene fluoride as the binder, an LiCoO2 powder as the positive-electrode raw-material powder and a carbon material as the negative-electrode raw-material powder, in some of the specific examples.    Patent Literature No. 1: Japanese Unexamined Patent Publication (KOKAI) Gazette No. 10-270013